This is a follow-up on #21480, which put src/setup.py in charge of all sagelibbuilding and removed dependencies on various environment variables, as a step towards the goal of making sagelib an ordinary Python package (#21507).

However, there is still a dependency on $SAGE_CYTHONIZED, which needs to be set in a way that matches the build-base (set by setup.py build --build-base).

This dependency should be removed.

Also, --inplace should be supported.
(See also: #12659: build the sage library in place)

See also #21508 on other cleanup issues of src/setup.py.

Depends on #21480
Depends on #21600
Depends on #21604
Depends on #23744

CC: @jdemeyer @embray

Component: build

Author: Jeroen Demeyer

Branch/Commit: bd91b43

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/21535